The exhaust gases from a motor vehicle's internal combustion engine have to be treated to ensure compliance with legally stipulated limits. The exhaust gases are for that purpose examined using exhaust sensors. Said exhaust sensors have to be kept at a specified operating temperature during operation. The exhaust sensors are for that purpose electrically heated by means of a temperature-regulated heater.
The exhaust sensors should be available ready to operate as soon as possible after the combustion engine starts and, to that end, should reach their operating temperature as quickly as possible. A voltage applied to a sensor heater is for that purpose increased as much as possible. The voltage applied to a sensor heater adheres in the prior art to a pre-fixed scheme. Switchover to temperature-regulated heating does not take place until the exhaust sensor's operating temperature has been reached. The scheme employed for the voltage curve on the sensor heater is a compromise between reaching the exhaust sensor's operational readiness as quickly as possible and preventing the exhaust sensor's destruction due to temperature changes that are too fast. To prevent the sensor's destruction, a maximum permissible temperature gradient in the exhaust sensor must not be exceeded, for example.
The fixed scheme for the heating-voltage curve takes no account of individual characteristics of the heating-voltage source or of the sensor heater employed. Because said characteristics vary in practice, the scheme for the heating-voltage curve must be devised such that the maximum permissible thermal load on the exhaust sensor will not be exceeded for any combinations of exhaust sensors and voltage-generating means. The disadvantage of that approach is that for most combinations of exhaust sensor and voltage-generating means the exhaust sensor is heated more slowly than is possible. Treatment for the exhaust gas after the combustion engine starts is consequently available later than is possible. In order nonetheless to fulfill the legal requirements placed on exhaust limits, the combustion engine's other systems must also be designed allowing for an appropriate reserve.
DE 10 2005 063 184 A1 describes a method for heating a gas sensor. The gas sensor is therein heated by means of pre-fixed heating voltages until measuring readiness has been attained. One or more switchover instants between the pre-fixed heating voltages can be ascertained as a function of the gas sensor's temperature. Changeover to a temperature-regulated heating strategy takes place when measuring readiness has been attained.
EP 1 239 282 A2 likewise describes a method for heating a gas sensor. The gas sensor is heated at a decreasing heating rate until the target temperature has been reached. Changeover to a regulated heating strategy takes place when the target temperature has been reached.